Radical New View of Role of Menstruation

By NATALIE ANGIER

Published: September 21, 1993

THE menstruating woman has been variously vilified, feared, pitied or banished from the village to spend her bloody days in solitude. Even the standard medical explanation connotes loss. A woman bleeds each month as a way of discarding her unfertilized eggs and the uterine lining that had been optimistically fattening up in anticipation of a baby that never arrived.

Now an evolutionary biologist proposes a radical new way of viewing menstruation, one that gives the ordinary business of having a period an active and salutary spin. The scientist, Margie Profet of the University of California at Berkeley, suggests that menstruation evolved as a mechanism for protecting a female's uterus and Fallopian tubes against harmful microbes delivered by incoming sperm.

According to this scheme, the uterus is extremely vulnerable to bacteria and viruses that may be hitching a ride on the sperm, and menstruation is an aggressive means of preventing infections that could lead to infertility, illness and even death. In menstruation, Ms. Profet suggests, the body takes a two-pronged attack against potential interlopers: it sloughs off the outer lining of the uterus, where the pathogens are likely to be lingering, and it bathes the area in blood, which carries immune cells to destroy the microbes.

"The body kills the tissue and ejects it, and it directly kills the pathogens with immune cells," said Ms. Profet. "It eliminates the pathogens and their home at the same time."

Ms. Profet presents her theory in a comprehensive report that takes up the bulk of the September issue of The Quarterly Review of Biology. In it, she seeks to answer the simple question of why the bodies of premenopausal women go to the trouble of shedding considerable quantities of blood and tissue each month, losing valuable nutrients, in particular iron, in the process.

Why not keep the uterine lining around until it is used, she wondered. And even if some of it must be turned over, why the messy bleeding?After all, the lining of the digestive tract is regenerated every two to four days, the skin sheds tens of thousands of cells every day, and other organs are freshened and patched up, all without the assistance of blood. "Menstruation is a costly event to the female, and it wouldn't be there if it didn't serve a very important purpose," she said.

Ms. Profet also suggests that other types of uterine bleeding, like that which sometimes accompanies ovulation and the implantation of the embryo and postpartum bleeding, may be the body's way of intermittently cleaning house and purging pathogenic intruders.

"It's an astonishing piece of work," said Dr. Donald Symons, a professor of anthropology and an evolutionary theorist at the University of California at Santa Barbara. "It's a fitting together of many disparate elements into one coherent explanatory system, and it's wonderful. It's exactly what a scientific theory should be."

Going further, in her article Ms. Profet says that humans and other higher primates are not the only mammals to menstruate, as is commonly supposed. Through an extensive review of scientific literature dating back to the last century, she has discovered that a number of mammals widely separated in evolutionary time have been observed to menstruate, including bats, marsupial cats, tree shrews and primitive monkeys. She predicts that it will turn out that nearly all mammals menstruate, if researchers only take the time to look, although many species may bleed only trace amounts that escape easy detection. "This is a bold prediction, and she's really going out on a limb in making it," said Dr. Symons.

Ms. Profet suggests that her hypothesis has important medical implications. If bleeding helps prevent infections, she said, then women should avoid oral contraceptives that suppress menstruation entirely. In addition, she said, inexplicable uterine bleeding should be viewed as a possible early sign of infection, a symptom that the body is struggling to thwart disease. Often doctors regard such bleeding as the result of abnormal hormonal flux, seeing it as a reaction that in turn increases a woman's risk of contracting a pelvic infection. But this attitude, Ms. Profet insists, is completely backwards.

"Saying uterine bleeding causes infections is like saying a fireman causes a fire," said Ms. Profet. If she is right, she said, the worst thing a doctor could do for an episode of unexplained uterine bleeding would be to block the bleeding with hormones. A more appropriate response, she said, might be to test for an infectious organism like chlamydia and then prescribe an immediate course of antibiotics.

The new hypothesis also may explain the puzzle of why women who use intrauterine devices have extremely heavy periods.

"The IUD causes chronic inflammation of the uterus, and in general inflammation is a sign of infection," said Ms. Profet. "My guess is the uterus thinks there's an infection there and increases blood flow."

There can be other reasons for unexplained bleeding -- for example, tumors, fibroid disease or ectopic pregnancy -- but Ms. Profet believes that infection should also be considered.